Phase-selection amplifiers are well suited for measuring weak signals in the presence of a high noise level. By mixing the signals to be measured with a reference voltage having the same frequency, mixture products are obtained that have a d.c. value only for the signal to be measured. This d.c. signal can therefore be easily filtered and further processed, while the signals having interfering frequencies have no d.c. component and therefore drop out. The signal to be measured is frequently a carrier that is amplitude-modulated with a low frequency signal. The modulation side bands formed must be processed by the phase-selective amplifier in spite of the high selectivity of the amplifier.
It is known to use a phase-selective amplifier, also referred to as a lock-in amplifier, for measuring weak signals, which may be referred to as desired signals, in the presence of a high noise level. After preamplification and filtering, the desired signal is supplied to one input terminal of a synchronous detector operating as a mixer stage. The second input terminal of the mixer stage is supplied with a rectangular-wave reference signal having a frequency corresponding to the known frequency of the desired signal to be measured. To equalize the phase, an adjustable phase shifter is provided in the reference channel. If the two input signals are in synchronism, alternate half-waves of the desired signal are inverted by the switching effect of the mixer stage, in step with the fixed cycle of the reference voltage. The inverted half-waves, along with the non-inverted half-waves, produce a d.c. signal component at the output of the mixer stage. This d.c. signal is further processed through low-pass filters and d.c. voltage amplifiers. Interfering frequencies, in general, are not synchronous with the reference signal and, therefore, produce no d.c. signal at the output of the mixer stage. An exception is a harmonic, particularly an odd harmonic, of the desired signal. Because of the rectangular shape of the reference voltage, which is equivalent to the summation of the fundamental wave with its odd harmonics, d.c. signals also appear at the output of the mixer stage due to these harmonics, which can be attenuated only by expensive filtering of the signal in advance of the mixer stage, narrowing the effective bandwidth (Electronic Design 21, Oct. 11, 1974, Vol. 22, pages 104-109).
The use of synchronous detectors in receivers for signals having a definite frequency are well known. The desired signal voltage is usually amplified and heterodyned to an intermediate frequency, and this i.f. signal is then applied to the input of a synchronous detector. An oscillator is synchronized, both as to frequency and phase, by a control voltage produced by comparing the output a.c. voltage of the oscillator with the signal a.c. voltage in a phase discriminator. The output voltage from the discriminator is used to control the oscillator to lock in on the received signal voltage in a fixed phase relation as long as the original frequency of the oscillator does not differ by more than a certain amount from the signal frequency. The range of frequencies within which this effect is produced is the so-called lock-in range.
The synchronized a.c. voltage of the oscillator is supplied to the reference input terminal of the synchronous detector, and a d.c. signal can appear at the output of the synchronous detector only if the received signal and the reference signal voltages applied to the inputs are substantially in phase. This condition is satisfied only by signal frequencies within the lock-in range of the oscillator and capable of synchronizing it. Consequently, the filtering effect of the arrangement is determined by the lock-in range of the oscillator. The synchronous detector is connected to a switch which, in turn, is controlled by the output signals of the detector. This switch causes a d.c. output of the signal receiver to be switched to high or low resistance to furnish the information as to whether or not the signal being received, is within the lock-in range of the oscillator.
The use of this prior art signal receiver is limited to signal levels sufficient for synchronizing the oscillator. Very weak signals accompanied by a higher noise level cannot be detected by supplying a predetermined reference voltage. Because of the lock-in range of the oscillator, the signal receiver is fixed to a predetermined frequency, and no provision is made for variable tuning to allow different frequencies to be received. (German AS No. 25 27 578).
Another prior art circuit for preventing the harmonics in the reference signal from affecting the output voltage of a phase-selective amplifier also comprises an a.c. signal voltage amplifier followed by a phase-selective rectifier which is controlled by a reference signal. The phase of the reference signal relative to that of the measured signal is known in advance, and the circuit produces a d.c. signal by utilizing the reference signal in an additional phase-selective rectifier. The two phase-selective rectifiers are controlled, either in phase or exactly in phase opposition or with the same phase shift by which the measured signal and the reference signal are out of phase. The d.c. output of the additional rectifier controls the multiplication factor of a multiplier which is connected in the negative-feedback circuit of the a.c. signal amplifier and determines the gain of the latter amplifier. As a result, the effect of the harmonics in the reference signal causes the feedback factor of the a.c. signal, given by the multiplication factor, to be controlled in proportion to the variation of the proportional control factor of the rectifier for the measured voltage. Therefore, with a satisfactorily linear gain of the a.c. signal amplifier, these two errors compensate each other. The phase-selective rectifier for the measured voltage is followed by another d.c. amplifier. This prior art circuit allows the phase-selective rectifier for the measured voltage to operate unaffected, in a conventional manner. The effect of the harmonics contained in the measured signal or of the corresponding interfering frequencies cannot be reduced (German AS No. 18 06 314).